Revert vertex position to Basis Shape Key
Use Blend From Shape:


Select all vertices whose positions you want to reset,
Execute Blend From Shape, accessible from W, Ctrl-V or Mesh > Vertices menu,
Set "Basis" as shape key to blend from (by default),
Set blend factor to the maximum value 1.0 (by default),
Uncheck Add, so only values from "Basis" will be blended in, effectively reverting all selected vertices' position.